Magnetic material heat treatment



Patented Jan. 5, 1943 v UNlTED STATE MAGNETIC MATERIAL HEAT TREATMENT William E. Ruder and Benjamin M. Smith, asslgnors to General Elec-v tric Company, a corporation of New York Schenectady, N. Y.,

No Drawing.

Application September 21, 1938,

Serial No. 231,017 3 Claims. (Cl. 175-356) The present invention relates to magnetic materials and more particularly to a method for improving the magnetic properties of silicon steel.

It is one of the objects of the present invention to obtain an appreciable reduction in hysteresis tion however may be applied with equal facility to hot rolled strip and to silicon steel sheets as well as to ferro-magnetic materials of the type disclosed in Patent No. 2,002,689, Bozorth et al.; Patent No. 2,002,696, Kelsall; and Patent No. 2,084,133, Dixon et al.

In carrying out the present invention high reduction cold rolled'silicon steel strip material containing about 3% to 4% silicon generally is annealed at about 850 C. to 900 C. preferably in a non-oxidizing atmosphere. While the steel may be annealed if desired at temperatures above 900 C. no appreciable improved results are obtained by the use of higher temperatures. After anneal the steel is cooled slowly, preferably in the furnace, to about room temperature and is thereafter reheated, preferably in a non-oxidizing atmosphere such as hydrogen, at a temperature about 50 to 100 C. above the magnetic transformation point. The steel is held at this temperature for a time suflicient to produce a uni- .form temperature throughout the steel, for example one hour or more, and then cooled, generally at a rate of about 100 C. per hour, to about room temperature. At the reheat temperature abovethe magnetic transformation point a magnetic field which may be either A.C. or D.C. is applied to the steel preferably in the same direction as that which is applied to the silicon steel when in use and is of such strength as would, at room temperature, bring the steel as close to the magnetic saturation point as is practical. The temperature of the steel is then slowly reduced, the magnetic field being maintained until the steel has passed through the non-magnetic state to a temperature of about 250 C. after which it may be removed, if desired, and the steel allowed to cool to room temperature. perature range for 3% silicon steel during which the magnetic field is applied is between 800 C. and 250 C., the most effective range being between 700" C. to 300 C. If desired the steps comprising reheating and cooling in a magnetic field The effective temmay be applied to steel which has been normalized .but not annealed or to steel which has been normalized as well as annealed.

While cold rolled silicon steel strip responds to fields as low as 60 oersteds, increased improvement may be obtained if the field strength is increased up to about 300 oersteds. Improvement in permeability in silicon steel obtained by our improved process is in the order of about 50% while the improvement obtained in hysteresis loss with our process is in the order of about 10% to 15% over the best results obtainable in like material which merely has been annealed.

The following table shows the effect of permeability and hysteresis losses for various materials treated by our process in A.-C. and D.--C. fields of varying strengths:

Hyst. atB= Maxb 10000 Magnetic Material mum. 'jf gs: fi gg oersteds Common iron Steel (0.4% $1) Hot rolled 3.5

. ewsw 0 t iPtd Heat treated without field. H=80, D.-C. H=80, A.-C.

Nickel-iron alloy 47% Ni, 53% Fe ll Cold rolled nickel-iron alloy 47% Ni. 53% Fe Silicon steel strip about .012 thick and containing about 3% silicon and treated in accordance with the present process has a permeability materially in excess of 20,000 and a total core loss of about 0.38 watt per pound for 10,000 gauss at 60 cycles.

Whil either an A.-C. or D.-C. magnetic field may be employed in our process, we prefer to use a D.-C. field since it gives slightly better results under some conditions than the A.-C. field. The field employed however should be of sufficient strength of about strength to carry netic saturation.

the induction up towards mag- As heretofore stated a field 60 to 80 oersteds provides -a substantial improvement in the magnetic qualities of the steel but in some material it may be found necessary to go to 300 oersteds to obtain the maximum improvement in magnetic properties.-

If the silicon steel to be treated is in the form of a strip wound transformer core, it is desirable that the convolutions should be free from oxide and tightly wound to reduce any air space between the convolutions to a minimum. A nonoxidizing atmosphere should be employed when annealing either the wound core or transformer punchings since an oxide-free material gives better magnetic properties per unit of volume. I

In magnetizing straight strips we prefer to employ a solenoid to eflect the magnetization, the solenoid having su-flicient ampere-turns to produce the required field. The solenoid may be built in as a part of the heat treating furnace. In the case of ring or completed core circuits, windings may be applied to the core but in practice this is not practical and it is preferable therefore to use a single conductor or, if necessary, one or more turns through which a very heavy current may be passed which is of sufficient strength to produce magnetic saturation or as near saturation as is practical. While we prefer to apply the magnetic field only during the period when the steel is cooled from the reheat temperature it obviously also may be applied while the steel is being reheated to the temperature above the magnetic transformation point.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method for improving the magnetic properties of ferro-magnetic material which comprises annealing between about 850 C. to 900 C., slowly cooling the material at a temperature the steel, reheating it to a temperature about to C. above the magnetic transformation Point, applying a D.-C. magnetic field to said material, said field having the strength of about a non-oxidizing atmosphere, cooling the core, reheating it in a non-oxidizing atmosphere at a temperature about 50 C. to 100 C. above its magnetic transformation point and cooling it in a magnetic field of sufficient strength to produce a condition approaching magnetic saturation in said core when at room temperature, said field being maintained at least while the core cools from about 700 C. to about 300 C. and having a direction the same as that employed when the transformer is in operation.

3. The method for improving the magnetic properties of a wound transformer core consisting of normalized silicon steel strip, said method comprising annealing said core in a non-oxidizing atmosphere, cooling the core, reheating it in a non-oxidizing atmosphere at a temperature about 50 C. to 100 0. above its magnetic transformation point and cooling it in a magnetic field of suflicient strength to produce a condition approaching magnetic saturation in said core when at room temperature, said field being maintained at least While the core cools from about 700 C. to about 300 C. and having a direction the same as that employed when the transformer is in operation.

WILLIAM E. RUDER. BENJAMIN M. SMITH. 

